SERVER-24415 Replace instances of shared_ptr<CollectionMetadata> with ScopedCollectionMetadata

18 files changed, 95 insertions, 73 deletions

diff --git a/src/mongo/db/commands/mr.cpp b/src/mongo/db/commands/mr.cpp
index ade9ae9225e..423cfeeec29 100644
--- a/src/mongo/db/commands/mr.cpp
+++ b/src/mongo/db/commands/mr.cpp
@@ -1358,10 +1358,9 @@ public:
 
         uassert(16149, "cannot run map reduce without the js engine", globalScriptEngine);
 
-        shared_ptr<CollectionMetadata> collMetadata;
-
         // Prevent sharding state from changing during the MR.
         unique_ptr<RangePreserver> rangePreserver;
+        ScopedCollectionMetadata collMetadata;
         {
             AutoGetCollectionForRead ctx(txn, config.ns);
 
diff --git a/src/mongo/db/exec/shard_filter.cpp b/src/mongo/db/exec/shard_filter.cpp
index 8388689cbc6..9c2f142622b 100644
--- a/src/mongo/db/exec/shard_filter.cpp
+++ b/src/mongo/db/exec/shard_filter.cpp
@@ -35,7 +35,7 @@
 #include "mongo/db/exec/filter.h"
 #include "mongo/db/exec/scoped_timer.h"
 #include "mongo/db/exec/working_set_common.h"
-#include "mongo/db/s/collection_metadata.h"
+#include "mongo/db/s/metadata_manager.h"
 #include "mongo/s/shard_key_pattern.h"
 #include "mongo/stdx/memory.h"
 #include "mongo/util/log.h"
@@ -51,10 +51,10 @@ using stdx::make_unique;
 const char* ShardFilterStage::kStageType = "SHARDING_FILTER";
 
 ShardFilterStage::ShardFilterStage(OperationContext* opCtx,
-                                   const shared_ptr<CollectionMetadata>& metadata,
+                                   ScopedCollectionMetadata metadata,
                                    WorkingSet* ws,
                                    PlanStage* child)
-    : PlanStage(kStageType, opCtx), _ws(ws), _metadata(metadata) {
+    : PlanStage(kStageType, opCtx), _ws(ws), _metadata(std::move(metadata)) {
     _children.emplace_back(child);
 }
 
diff --git a/src/mongo/db/exec/shard_filter.h b/src/mongo/db/exec/shard_filter.h
index 44d7bc08055..0a180afd2e6 100644
--- a/src/mongo/db/exec/shard_filter.h
+++ b/src/mongo/db/exec/shard_filter.h
@@ -29,11 +29,10 @@
 #pragma once
 
 #include "mongo/db/exec/plan_stage.h"
+#include "mongo/db/s/metadata_manager.h"
 
 namespace mongo {
 
-class CollectionMetadata;
-
 /**
  * This stage drops documents that didn't belong to the shard we're executing on at the time of
  * construction. This matches the contract for sharded cursorids which guarantees that a
@@ -72,7 +71,7 @@ class CollectionMetadata;
 class ShardFilterStage final : public PlanStage {
 public:
     ShardFilterStage(OperationContext* opCtx,
-                     const std::shared_ptr<CollectionMetadata>& metadata,
+                     ScopedCollectionMetadata metadata,
                      WorkingSet* ws,
                      PlanStage* child);
     ~ShardFilterStage();
@@ -98,7 +97,7 @@ private:
 
     // Note: it is important that this is the metadata from the time this stage is constructed.
     // See class comment for details.
-    const std::shared_ptr<CollectionMetadata> _metadata;
+    ScopedCollectionMetadata _metadata;
 };
 
 }  // namespace mongo
diff --git a/src/mongo/db/exec/update.cpp b/src/mongo/db/exec/update.cpp
index 7981c55cc23..a8f58613a5b 100644
--- a/src/mongo/db/exec/update.cpp
+++ b/src/mongo/db/exec/update.cpp
@@ -436,8 +436,7 @@ bool shouldRestartUpdateIfNoLongerMatches(const UpdateStageParams& params) {
 };
 
 const std::vector<FieldRef*>* getImmutableFields(OperationContext* txn, const NamespaceString& ns) {
-    std::shared_ptr<CollectionMetadata> metadata =
-        CollectionShardingState::get(txn, ns)->getMetadata();
+    auto metadata = CollectionShardingState::get(txn, ns)->getMetadata();
     if (metadata) {
         const std::vector<FieldRef*>& fields = metadata->getKeyPatternFields();
         // Return shard-keys as immutable for the update system.
diff --git a/src/mongo/db/query/get_executor.cpp b/src/mongo/db/query/get_executor.cpp
index b4e9ca6de89..2e9189c87e3 100644
--- a/src/mongo/db/query/get_executor.cpp
+++ b/src/mongo/db/query/get_executor.cpp
@@ -175,8 +175,7 @@ void fillOutPlannerParams(OperationContext* txn,
 
     // If the caller wants a shard filter, make sure we're actually sharded.
     if (plannerParams->options & QueryPlannerParams::INCLUDE_SHARD_FILTER) {
-        std::shared_ptr<CollectionMetadata> collMetadata =
-            CollectionShardingState::get(txn, canonicalQuery->nss())->getMetadata();
+        auto collMetadata = CollectionShardingState::get(txn, canonicalQuery->nss())->getMetadata();
         if (collMetadata) {
             plannerParams->shardKey = collMetadata->getKeyPattern();
         } else {
diff --git a/src/mongo/db/s/cleanup_orphaned_cmd.cpp b/src/mongo/db/s/cleanup_orphaned_cmd.cpp
index ff27007fb77..ea04f994a59 100644
--- a/src/mongo/db/s/cleanup_orphaned_cmd.cpp
+++ b/src/mongo/db/s/cleanup_orphaned_cmd.cpp
@@ -80,7 +80,7 @@ CleanupResult cleanupOrphanedData(OperationContext* txn,
                                   string* errMsg) {
     BSONObj startingFromKey = startingFromKeyConst;
 
-    std::shared_ptr<CollectionMetadata> metadata;
+    ScopedCollectionMetadata metadata;
     {
         AutoGetCollection autoColl(txn, ns, MODE_IS);
         metadata = CollectionShardingState::get(txn, ns.toString())->getMetadata();
@@ -116,9 +116,6 @@ CleanupResult cleanupOrphanedData(OperationContext* txn,
     orphanRange.ns = ns.ns();
     *stoppedAtKey = orphanRange.maxKey;
 
-    // We're done with this metadata now, no matter what happens
-    metadata.reset();
-
     LOG(1) << "orphaned data cleanup requested for " << ns.toString() << " starting from "
            << startingFromKey << ", removing next orphan range"
            << " [" << orphanRange.minKey << "," << orphanRange.maxKey << ")";
diff --git a/src/mongo/db/s/collection_sharding_state.cpp b/src/mongo/db/s/collection_sharding_state.cpp
index 29606ca8640..af9e67fdbfe 100644
--- a/src/mongo/db/s/collection_sharding_state.cpp
+++ b/src/mongo/db/s/collection_sharding_state.cpp
@@ -78,7 +78,9 @@ using std::string;
 
 CollectionShardingState::CollectionShardingState(
     NamespaceString nss, std::unique_ptr<CollectionMetadata> initialMetadata)
-    : _nss(std::move(nss)), _metadata(std::move(initialMetadata)) {}
+    : _nss(std::move(nss)), _metadataManager{} {
+    _metadataManager.setActiveMetadata(std::move(initialMetadata));
+}
 
 CollectionShardingState::~CollectionShardingState() {
     invariant(!_sourceMgr);
@@ -98,13 +100,16 @@ CollectionShardingState* CollectionShardingState::get(OperationContext* txn,
     return shardingState->getNS(ns);
 }
 
-void CollectionShardingState::setMetadata(std::shared_ptr<CollectionMetadata> newMetadata) {
+ScopedCollectionMetadata CollectionShardingState::getMetadata() {
+    return _metadataManager.getActiveMetadata();
+}
+
+void CollectionShardingState::setMetadata(std::unique_ptr<CollectionMetadata> newMetadata) {
     if (newMetadata) {
         invariant(!newMetadata->getCollVersion().isWriteCompatibleWith(ChunkVersion::UNSHARDED()));
         invariant(!newMetadata->getShardVersion().isWriteCompatibleWith(ChunkVersion::UNSHARDED()));
     }
-
-    _metadata = std::move(newMetadata);
+    _metadataManager.setActiveMetadata(std::move(newMetadata));
 }
 
 MigrationSourceManager* CollectionShardingState::getMigrationSourceManager() {
@@ -127,7 +132,7 @@ void CollectionShardingState::clearMigrationSourceManager(OperationContext* txn)
     _sourceMgr = nullptr;
 }
 
-void CollectionShardingState::checkShardVersionOrThrow(OperationContext* txn) const {
+void CollectionShardingState::checkShardVersionOrThrow(OperationContext* txn) {
     string errmsg;
     ChunkVersion received;
     ChunkVersion wanted;
@@ -204,7 +209,7 @@ void CollectionShardingState::onDeleteOp(OperationContext* txn, const BSONObj& d
 bool CollectionShardingState::_checkShardVersionOk(OperationContext* txn,
                                                    string* errmsg,
                                                    ChunkVersion* expectedShardVersion,
-                                                   ChunkVersion* actualShardVersion) const {
+                                                   ChunkVersion* actualShardVersion) {
     Client* client = txn->getClient();
 
     // Operations using the DBDirectClient are unversioned.
@@ -240,7 +245,8 @@ bool CollectionShardingState::_checkShardVersionOk(OperationContext* txn,
     }
 
     // Set this for error messaging purposes before potentially returning false.
-    *actualShardVersion = (_metadata ? _metadata->getShardVersion() : ChunkVersion::UNSHARDED());
+    auto metadata = getMetadata();
+    *actualShardVersion = metadata ? metadata->getShardVersion() : ChunkVersion::UNSHARDED();
 
     if (_sourceMgr && _sourceMgr->getMigrationCriticalSectionSignal()) {
         *errmsg = str::stream() << "migration commit in progress for " << _nss.ns();
diff --git a/src/mongo/db/s/collection_sharding_state.h b/src/mongo/db/s/collection_sharding_state.h
index 449c8b4ba6b..ba482b15c74 100644
--- a/src/mongo/db/s/collection_sharding_state.h
+++ b/src/mongo/db/s/collection_sharding_state.h
@@ -34,6 +34,7 @@
 #include "mongo/base/disallow_copying.h"
 #include "mongo/base/string_data.h"
 #include "mongo/db/namespace_string.h"
+#include "mongo/db/s/metadata_manager.h"
 
 namespace mongo {
 
@@ -75,14 +76,12 @@ public:
     /**
      * Returns the chunk metadata for the collection.
      */
-    std::shared_ptr<CollectionMetadata> getMetadata() const {
-        return _metadata;
-    }
+    ScopedCollectionMetadata getMetadata();
 
     /**
      * Set a new metadata to be used for this collection.
      */
-    void setMetadata(std::shared_ptr<CollectionMetadata> newMetadata);
+    void setMetadata(std::unique_ptr<CollectionMetadata> newMetadata);
 
     /**
      * Returns the active migration source manager, if one is available.
@@ -112,7 +111,7 @@ public:
      * response is constructed, this function should be the only means of checking for shard version
      * match.
      */
-    void checkShardVersionOrThrow(OperationContext* txn) const;
+    void checkShardVersionOrThrow(OperationContext* txn);
 
     // Replication subsystem hooks. If this collection is serving as a source for migration, these
     // methods inform it of any changes to its contents.
@@ -142,14 +141,12 @@ private:
     bool _checkShardVersionOk(OperationContext* txn,
                               std::string* errmsg,
                               ChunkVersion* expectedShardVersion,
-                              ChunkVersion* actualShardVersion) const;
+                              ChunkVersion* actualShardVersion);
 
     // Namespace to which this state belongs.
     const NamespaceString _nss;
 
-    // Contains all the chunks associated with this collection. This value will be null if the
-    // collection is not sharded.
-    std::shared_ptr<CollectionMetadata> _metadata;
+    MetadataManager _metadataManager;
 
     // If this collection is serving as a source shard for chunk migration, this value will be
     // non-null. To write this value there needs to be X-lock on the collection in order to
diff --git a/src/mongo/db/s/get_shard_version_command.cpp b/src/mongo/db/s/get_shard_version_command.cpp
index 1331bbde061..5e5e1edef97 100644
--- a/src/mongo/db/s/get_shard_version_command.cpp
+++ b/src/mongo/db/s/get_shard_version_command.cpp
@@ -114,7 +114,11 @@ public:
         AutoGetCollection autoColl(txn, nss, MODE_IS);
         CollectionShardingState* const css = CollectionShardingState::get(txn, nss);
 
-        shared_ptr<CollectionMetadata> metadata(css ? css->getMetadata() : nullptr);
+        ScopedCollectionMetadata metadata;
+        if (css) {
+            metadata = css->getMetadata();
+        }
+
         if (metadata) {
             result.appendTimestamp("global", metadata->getShardVersion().toLong());
         } else {
diff --git a/src/mongo/db/s/merge_chunks_command.cpp b/src/mongo/db/s/merge_chunks_command.cpp
index dd688648be8..f996fda63e9 100644
--- a/src/mongo/db/s/merge_chunks_command.cpp
+++ b/src/mongo/db/s/merge_chunks_command.cpp
@@ -39,6 +39,7 @@
 #include "mongo/db/namespace_string.h"
 #include "mongo/db/s/collection_metadata.h"
 #include "mongo/db/s/collection_sharding_state.h"
+#include "mongo/db/s/metadata_manager.h"
 #include "mongo/db/s/sharding_state.h"
 #include "mongo/s/catalog/type_chunk.h"
 #include "mongo/s/grid.h"
@@ -196,7 +197,7 @@ bool mergeChunks(OperationContext* txn,
         return false;
     }
 
-    std::shared_ptr<CollectionMetadata> metadata;
+    ScopedCollectionMetadata metadata;
     {
         AutoGetCollection autoColl(txn, nss, MODE_IS);
 
diff --git a/src/mongo/db/s/metadata_manager.cpp b/src/mongo/db/s/metadata_manager.cpp
index cf2cb76def5..457d1e9ae29 100644
--- a/src/mongo/db/s/metadata_manager.cpp
+++ b/src/mongo/db/s/metadata_manager.cpp
@@ -43,7 +43,9 @@ MetadataManager::~MetadataManager() {
 
 ScopedCollectionMetadata MetadataManager::getActiveMetadata() {
     stdx::lock_guard<stdx::mutex> scopedLock(_managerLock);
-    invariant(_activeMetadataTracker);
+    if (!_activeMetadataTracker) {
+        return ScopedCollectionMetadata();
+    }
     return ScopedCollectionMetadata(this, _activeMetadataTracker.get());
 }
 
@@ -72,6 +74,8 @@ MetadataManager::CollectionMetadataTracker::CollectionMetadataTracker(
     std::unique_ptr<CollectionMetadata> m)
     : metadata(std::move(m)), usageCounter(0){};
 
+ScopedCollectionMetadata::ScopedCollectionMetadata() = default;
+
 // called in lock
 ScopedCollectionMetadata::ScopedCollectionMetadata(
     MetadataManager* manager, MetadataManager::CollectionMetadataTracker* tracker)
@@ -80,6 +84,9 @@ ScopedCollectionMetadata::ScopedCollectionMetadata(
 }
 
 ScopedCollectionMetadata::~ScopedCollectionMetadata() {
+    if (!_tracker)
+        return;
+
     stdx::lock_guard<stdx::mutex> scopedLock(_manager->_managerLock);
     invariant(_tracker->usageCounter > 0);
     if (--_tracker->usageCounter == 0) {
@@ -110,6 +117,10 @@ ScopedCollectionMetadata& ScopedCollectionMetadata::operator=(ScopedCollectionMe
     return *this;
 }
 
+ScopedCollectionMetadata::operator bool() const {
+    return _tracker && _tracker->metadata.get();
+}
+
 std::map<BSONObj, ChunkRange> MetadataManager::getCopyOfRanges() {
     stdx::lock_guard<stdx::mutex> scopedLock(_managerLock);
     return _rangesToClean;
diff --git a/src/mongo/db/s/metadata_manager.h b/src/mongo/db/s/metadata_manager.h
index 73f91f7e44f..04135cef840 100644
--- a/src/mongo/db/s/metadata_manager.h
+++ b/src/mongo/db/s/metadata_manager.h
@@ -123,6 +123,12 @@ class ScopedCollectionMetadata {
 
 public:
     /**
+     * Creates an empty ScopedCollectionMetadata. Using the default constructor means that no
+     * metadata is available.
+     */
+    ScopedCollectionMetadata();
+
+    /**
      * Decrements the usageCounter and conditionally makes a call to _removeMetadata on
      * the tracker if the count has reached zero.
      */
@@ -137,6 +143,9 @@ public:
     CollectionMetadata* operator->();
     CollectionMetadata* getMetadata();
 
+    /** True if the ScopedCollectionMetadata stores a metadata (is not empty) */
+    operator bool() const;
+
 private:
     friend ScopedCollectionMetadata MetadataManager::getActiveMetadata();
 
@@ -146,8 +155,8 @@ private:
     ScopedCollectionMetadata(MetadataManager* manager,
                              MetadataManager::CollectionMetadataTracker* tracker);
 
-    MetadataManager* _manager;
-    MetadataManager::CollectionMetadataTracker* _tracker;
+    MetadataManager* _manager{nullptr};
+    MetadataManager::CollectionMetadataTracker* _tracker{nullptr};
 };
 
 }  // namespace mongo
diff --git a/src/mongo/db/s/migration_source_manager.cpp b/src/mongo/db/s/migration_source_manager.cpp
index 61a4399a7c5..cdc188dcd46 100644
--- a/src/mongo/db/s/migration_source_manager.cpp
+++ b/src/mongo/db/s/migration_source_manager.cpp
@@ -120,8 +120,8 @@ MigrationSourceManager::MigrationSourceManager(OperationContext* txn, MoveChunkR
         ScopedTransaction scopedXact(txn, MODE_IS);
         AutoGetCollection autoColl(txn, _args.getNss(), MODE_IS);
 
-        auto css = CollectionShardingState::get(txn, _args.getNss());
-        _committedMetadata = css->getMetadata();
+        _committedMetadata = CollectionShardingState::get(txn, _args.getNss())->getMetadata();
+        _keyPattern = _committedMetadata->getKeyPattern();
     }
 
     const ChunkVersion collectionVersion = _committedMetadata->getCollVersion();
@@ -357,10 +357,10 @@ Status MigrationSourceManager::commitDonateChunk(OperationContext* txn) {
         ChunkType migratingChunkToForget;
         migratingChunkToForget.setMin(_args.getMinKey());
         migratingChunkToForget.setMax(_args.getMaxKey());
-        _committedMetadata =
-            _committedMetadata->cloneMigrate(migratingChunkToForget, uncommittedCollVersion);
         auto css = CollectionShardingState::get(txn, _args.getNss().ns());
-        css->setMetadata(_committedMetadata);
+        css->setMetadata(
+            _committedMetadata->cloneMigrate(migratingChunkToForget, uncommittedCollVersion));
+        _committedMetadata = css->getMetadata();
     } else {
         // This could be an unrelated error (e.g. network error). Check whether the metadata update
         // succeeded by refreshing the collection metadata from the config server and checking that
@@ -415,8 +415,8 @@ Status MigrationSourceManager::commitDonateChunk(OperationContext* txn) {
             AutoGetCollection autoColl(txn, _args.getNss(), MODE_IS);
 
             ChunkVersion previousMetadataCollVersion = _committedMetadata->getCollVersion();
-            auto css = CollectionShardingState::get(txn, _args.getNss());
-            std::shared_ptr<CollectionMetadata> refreshedMetadata = css->getMetadata();
+            auto refreshedMetadata =
+                CollectionShardingState::get(txn, _args.getNss())->getMetadata();
 
             if (refreshedMetadata->keyBelongsToMe(_args.getMinKey())) {
                 invariant(refreshedMetadata->getCollVersion() == previousMetadataCollVersion);
diff --git a/src/mongo/db/s/migration_source_manager.h b/src/mongo/db/s/migration_source_manager.h
index 16eddc2351b..a5f1260c098 100644
--- a/src/mongo/db/s/migration_source_manager.h
+++ b/src/mongo/db/s/migration_source_manager.h
@@ -31,6 +31,7 @@
 #include <string>
 
 #include "mongo/base/disallow_copying.h"
+#include "mongo/db/s/metadata_manager.h"
 #include "mongo/s/move_chunk_request.h"
 #include "mongo/s/shard_key_pattern.h"
 #include "mongo/util/concurrency/notification.h"
@@ -38,7 +39,6 @@
 
 namespace mongo {
 
-class CollectionMetadata;
 class MigrationChunkClonerSource;
 class OperationContext;
 
@@ -145,12 +145,10 @@ public:
     void cleanupOnError(OperationContext* txn);
 
     /**
-     * Retrieves the last known committed collection metadata. What gets returned by this call may
-     * change before and after the critical section has started. It should only be used for
-     * diagnostics purposes and not relied on for any routing/consistency checking decisions.
+     * Returns the key pattern object for the stored committed metadata.
      */
-    std::shared_ptr<CollectionMetadata> getCommittedMetadata() const {
-        return _committedMetadata;
+    BSONObj getKeyPattern() const {
+        return _keyPattern;
     }
 
     /**
@@ -197,7 +195,10 @@ private:
     // The cached collection metadata from just after the collection distributed lock was acquired.
     // This metadata is guaranteed to not change until either failure or successful completion,
     // because the distributed lock is being held. Available after stabilize stage has completed.
-    std::shared_ptr<CollectionMetadata> _committedMetadata;
+    ScopedCollectionMetadata _committedMetadata;
+
+    // The key pattern of the collection whose chunks are being moved.
+    BSONObj _keyPattern;
 
     // The chunk cloner source. Only available if there is an active migration going on. To set and
     // remove it, global S lock needs to be acquired first in order to block all logOp calls and
diff --git a/src/mongo/db/s/move_chunk_command.cpp b/src/mongo/db/s/move_chunk_command.cpp
index e7f332fe9f3..493bf35ad3d 100644
--- a/src/mongo/db/s/move_chunk_command.cpp
+++ b/src/mongo/db/s/move_chunk_command.cpp
@@ -224,8 +224,7 @@ private:
 
             MigrationSourceManager migrationSourceManager(txn, moveChunkRequest);
 
-            shardKeyPattern =
-                migrationSourceManager.getCommittedMetadata()->getKeyPattern().getOwned();
+            shardKeyPattern = migrationSourceManager.getKeyPattern().getOwned();
 
             moveTimingHelper.done(2);
             MONGO_FAIL_POINT_PAUSE_WHILE_SET(moveChunkHangAtStep2);
diff --git a/src/mongo/db/s/sharding_state.cpp b/src/mongo/db/s/sharding_state.cpp
index b810a710d0b..f54eafc6ae4 100644
--- a/src/mongo/db/s/sharding_state.cpp
+++ b/src/mongo/db/s/sharding_state.cpp
@@ -270,8 +270,7 @@ void ShardingState::clearCollectionMetadata() {
 }
 
 ChunkVersion ShardingState::getVersion(const string& ns) {
-    shared_ptr<CollectionMetadata> p;
-
+    ScopedCollectionMetadata p;
     {
         stdx::lock_guard<stdx::mutex> lk(_mutex);
         CollectionShardingStateMap::const_iterator it = _collections.find(ns);
@@ -319,8 +318,7 @@ Status ShardingState::onStaleShardVersion(OperationContext* txn,
     {
         AutoGetCollection autoColl(txn, nss, MODE_IS);
 
-        shared_ptr<CollectionMetadata> storedMetadata =
-            CollectionShardingState::get(txn, nss)->getMetadata();
+        auto storedMetadata = CollectionShardingState::get(txn, nss)->getMetadata();
         if (storedMetadata) {
             collectionShardVersion = storedMetadata->getShardVersion();
         }
@@ -647,7 +645,7 @@ Status ShardingState::_refreshMetadata(OperationContext* txn,
     // The idea here is that we're going to reload the metadata from the config server, but we need
     // to do so outside any locks.  When we get our result back, if the current metadata has
     // changed, we may not be able to install the new metadata.
-    shared_ptr<CollectionMetadata> beforeMetadata;
+    ScopedCollectionMetadata beforeMetadata;
     {
         ScopedTransaction transaction(txn, MODE_IS);
         AutoGetCollection autoColl(txn, nss, MODE_IS);
@@ -698,12 +696,13 @@ Status ShardingState::_refreshMetadata(OperationContext* txn,
     long long refreshMillis;
 
     {
-        Status status = mdLoader.makeCollectionMetadata(txn,
-                                                        grid.catalogClient(txn),
-                                                        ns,
-                                                        getShardName(),
-                                                        fullReload ? nullptr : beforeMetadata.get(),
-                                                        remoteMetadata.get());
+        Status status =
+            mdLoader.makeCollectionMetadata(txn,
+                                            grid.catalogClient(txn),
+                                            ns,
+                                            getShardName(),
+                                            fullReload ? nullptr : beforeMetadata.getMetadata(),
+                                            remoteMetadata.get());
         refreshMillis = refreshTimer.millis();
 
         if (status.code() == ErrorCodes::NamespaceNotFound) {
@@ -727,7 +726,7 @@ Status ShardingState::_refreshMetadata(OperationContext* txn,
     // Get ready to install loaded metadata if needed
     //
 
-    shared_ptr<CollectionMetadata> afterMetadata;
+    ScopedCollectionMetadata afterMetadata;
     ChunkVersion afterShardVersion;
     ChunkVersion afterCollVersion;
     VersionChoice choice;
@@ -750,6 +749,7 @@ Status ShardingState::_refreshMetadata(OperationContext* txn,
         // Get the metadata now that the load has completed
         auto css = CollectionShardingState::get(txn, nss);
         afterMetadata = css->getMetadata();
+
         if (afterMetadata) {
             afterShardVersion = afterMetadata->getShardVersion();
             afterCollVersion = afterMetadata->getCollVersion();
@@ -761,7 +761,8 @@ Status ShardingState::_refreshMetadata(OperationContext* txn,
         // Resolve newer pending chunks with the remote metadata, finish construction
         //
 
-        Status status = mdLoader.promotePendingChunks(afterMetadata.get(), remoteMetadata.get());
+        Status status =
+            mdLoader.promotePendingChunks(afterMetadata.getMetadata(), remoteMetadata.get());
         if (!status.isOK()) {
             warning() << "remote metadata for " << ns
                       << " is inconsistent with current pending chunks"
@@ -894,7 +895,7 @@ void ShardingState::appendInfo(OperationContext* txn, BSONObjBuilder& builder) {
     for (CollectionShardingStateMap::const_iterator it = _collections.begin();
          it != _collections.end();
          ++it) {
-        shared_ptr<CollectionMetadata> metadata = it->second->getMetadata();
+        ScopedCollectionMetadata metadata = it->second->getMetadata();
         if (metadata) {
             versionB.appendTimestamp(it->first, metadata->getShardVersion().toLong());
         } else {
@@ -917,7 +918,7 @@ bool ShardingState::needCollectionMetadata(OperationContext* txn, const string&
         OperationShardingState::get(txn).hasShardVersion();
 }
 
-shared_ptr<CollectionMetadata> ShardingState::getCollectionMetadata(const string& ns) {
+ScopedCollectionMetadata ShardingState::getCollectionMetadata(const string& ns) {
     stdx::lock_guard<stdx::mutex> lk(_mutex);
 
     CollectionShardingStateMap::const_iterator it = _collections.find(ns);
diff --git a/src/mongo/db/s/sharding_state.h b/src/mongo/db/s/sharding_state.h
index 5b12c44d19d..4a53906370a 100644
--- a/src/mongo/db/s/sharding_state.h
+++ b/src/mongo/db/s/sharding_state.h
@@ -48,10 +48,10 @@ namespace mongo {
 class BSONObj;
 class BSONObjBuilder;
 struct ChunkVersion;
-class CollectionMetadata;
 class CollectionShardingState;
 class ConnectionString;
 class OperationContext;
+class ScopedCollectionMetadata;
 class ServiceContext;
 class ShardIdentityType;
 class Status;
@@ -200,7 +200,7 @@ public:
 
     bool needCollectionMetadata(OperationContext* txn, const std::string& ns);
 
-    std::shared_ptr<CollectionMetadata> getCollectionMetadata(const std::string& ns);
+    ScopedCollectionMetadata getCollectionMetadata(const std::string& ns);
 
     /**
      * Updates the config server field of the shardIdentity document with the given connection
diff --git a/src/mongo/db/s/split_chunk_command.cpp b/src/mongo/db/s/split_chunk_command.cpp
index f9c1a56710a..90dc2c96427 100644
--- a/src/mongo/db/s/split_chunk_command.cpp
+++ b/src/mongo/db/s/split_chunk_command.cpp
@@ -282,7 +282,7 @@ public:
             }
         }
 
-        std::shared_ptr<CollectionMetadata> collMetadata;
+        ScopedCollectionMetadata collMetadata;
         {
             AutoGetCollection autoColl(txn, nss, MODE_IS);
 
@@ -291,7 +291,7 @@ public:
         }
 
         // With nonzero shard version, we must have metadata
-        invariant(NULL != collMetadata);
+        invariant(collMetadata);
 
         ChunkVersion collVersion = collMetadata->getCollVersion();
         // With nonzero shard version, we must have a coll version >= our shard version